Valves, such as proportional pressure control valves, are very commonly used in mobile work machines for electrohydraulic pilot control of directional valves. One example of such a valve is disclosed in WO 2010/085991 A2.
For “smaller devices”, directly controlled proportional pressure control valves are usually sufficient. However, in the case of large construction machines, for example earth-moving machines such as diggers, the directional valves of the working hydraulics reach piston diameters at which pilot control with directly controlled proportional pressure control valves is no longer practical. The large slide valves produce during rapid switching a flow of pilot oil that can no longer be tolerated by the small pilot valves.
For such applications, proportional pressure control valves with corresponding surface ratios are therefore used. These valves have a pressure-effective surface smaller than the piston diameter. The force produced by the control pressure, which acts against the magnetic system usually in the form of an actuating magnet, is then significantly reduced. The large piston can, however, allow the passage of a significantly larger volume flow compared with the directly controlled valve, and thus, drastically reduce, in a desirable manner, the switching times of the large slide valves.
This principle of surface ratios has the significant advantage over a conventional pilot-controlled proportional pressure control valve in that a continuous pilot oil flow is not necessary. The leakage of these valves is extremely low in particular in the flow-free state. This characteristic has significant advantages in the case of emergency supply of the proportional valves with a pressure accumulator. However, a fundamental problem then exists when “filling” the directional valve piston to be actuated. In order to be able to achieve the coverage of the directional valve piston as quickly as possible, specifically when the function is started, the proportional valve is briefly flowed through with a relatively large volume flow. The proportional valve may be switched up to the stroke stop. If the desired slide position is then achieved, the directional piston must however also be held in the achieved position; and the pilot oil flow must consequently abruptly drop to zero. The proportional valve must therefore then move out of its end position back into a central position, whereby the control edge between the pressure supply connection P or pump connection and the utility connection or working connection A is then shut.
Irregularities can arise with the solutions of the prior art. In the stroke end position, the valve piston of the valve is at its mechanical stroke stop. Once this stroke stop has been reached, the dither flow, which is standard in proportional technology, then no longer has the opportunity to transfer the microvibrations of the pilot piston of the actuating magnet, which are standard in other operation, to the valve piston, and thus, to minimize its friction. The dither voltage or the dither flow is, as a dither signal, superimposed on the analogue actuating signal of the actuating magnet for the actuation of the valve piston of the valve. This signal then causes the valve or its piston to always vibrate slightly, so that no stick-slip effect occurs. As a general rule, setting the dither frequency as the frequency of the dither voltage should be possible just like the amplitude of the dither voltage or of the dither flow. Incorrectly set dither signal values lead to rapid wear of the valves. If the valve piston is kept at least briefly in its position inside the valve housing due to increased friction caused by the corresponding absence of the required dither signal from the actuating magnet as a result of increased friction, functional impairments are encountered, in particular in the form of jerky movements of the work machine, which represents a safety hazard.